This is a proposal from a group of seven NIH funded principal investigators requesting funds to purchase a commercially available two- photon only microscope system. This device will permit high-resolution optical images of structure and function of various cell types in living mice and zebrafish. The proposed studies include developmental biology, neurobiology and immunology projects. In particular, we propose monitoring the dynamics of radial glia proliferation in embryonic mouse forebrain, analyzing cell fate specification during zebrafish embryogenesis, imaging the development of electroresponsive and signaling properties of zebrafish oculomoter neurons, and determining structural alterations of synapses in transgenic mouse models of Alzheimer's disease. In the immune system we propose monitoring T cell interactions with the immunosuppressive tumor microenvironment, as well as with antigen presenting dendritic cells in the cervical lymph nodes of living mice in the context of normal immune responses and autoimmune disease. For each of the projects, the requested two-photon system will provide valuable new information difficult or nearly impossible to obtain with traditional confocal imaging approaches. The key advantage that is cited through these studies is the requirement to image up to 400 mm deep in tissues. The device will be housed in the animal facility of the Skirball Institute, a shared facility close to all seven users' laboratories and will also be available to the research community at New York University as time permits. This device will have long-term institutional support to cover up the service contracts as well as part of a technician's salary.